Signal transfer method and apparatus

ABSTRACT

An x-y digitising system is described which operates with a resonant stylus. The x-y digitising system includes an excitation winding for energising the resonant stylus and a number of sensor windings for receiving a signal re-radiated by the resonant stylus when energised. At least one of the excitation winding and the sensor winding is arranged to have its effective magnetic axis non-othogonal to the working area of the x-y digitising system. With this arrangement, improved energy transfer between the x-y digitising system and the resonant stylus can be achieved and/or improved position measurement accuracy can be obtained.

The present invention relates to an apparatus for and a method oftransferring signals between two devices. The invention has particularapplication in portable, battery-powered devices such as personaldigital assistants, mobile telephones, tablet PCs, web browsers, etc.

Inductive digitiser systems used in portable computer devices work bytransferring power to a moveable stylus inductively. When powered, thestylus in turn transmits a signal back to the digitiser tablet and thisis detected by a number of detecting elements to determine the positionand status of the stylus. The return signal power needs to besufficiently high relative to noise sources to yield position and statusinformation acceptably free from noise, such as position jitter.Additionally, it may also be desirable to power electronic circuitry inthe stylus, which requires additional power. The digitiser tablet musttherefore emit sufficient magnetic field to provide this power to thestylus. Where the power source is a battery, it is desirable to emitthis magnetic field efficiently, using as small an amount of power fromthe battery as possible, within the constraints of cost and space.

There are a number of existing digitiser systems that inductively powera stylus. U.S. Pat. No. 4,878,553 discloses a system that powers thestylus using an array of loop coils or windings which are arrayed overan x and y direction. The system powers the stylus by passing currentthrough those loop coils in closest proximity to the stylus. However,this arrangement is highly inefficient because there are a large numberof loop coils, each of which is formed from a small number of conductorloops printed on a printed circuit board (PCB) and therefore the widthof copper available for each loop coil can only be small. Additionally,the printed circuit board process itself limits conductor thickness totypically 0.05 mm, so that the overall volume of copper per loop isrelatively small. As is well-known to those skilled in the art, such asmall volume of copper results in the system being relativelyinefficient at powering the stylus when it is above the coil.

WO 00/33244 describes another digitiser system for use in a personaldigital assistant or a mobile telephone, where the stylus is powered bya single printed winding that is wound around the perimeter of theprinted circuit board. In this system, a larger proportion of theprinted circuit board area is devoted to this excitation winding,yielding greater copper mass and hence efficiency. However, the printedcircuit board is usually installed behind a display with a metal bezeland a layer of magnetic screening material is provided to shield thedigitiser signals from processing electronics beneath the printedcircuit board. With this arrangement, eddy current losses in both thescreening material and the bezel represent significant sources of powerloss. Although the bezel may be slit to prevent such eddy currentlosses, this is undesirable due to a weakened display housing, thegreater potential for electrostatic damage to the display and the needfor customised display mechanics.

Another problem experienced by this type of stylus digitiser system isthe tilting of the stylus by the user during use, which results in aposition offset in the position measurement. Most current systems try toovercome this problem by using dedicated algorithms which process thesignals from the loop coils or sensor coils to determine the tilt andhence the position offset error. However, these tilt correctionalgorithms rely on signals from conductors which are placed on eitherside of the actual position of the stylus and work well in the centre ofthe measurement area. However, when the stylus is positioned over anedge of the measurement area, the tilt correction algorithms become lessaccurate (because windings are only available to one side of the stylus)resulting in less accurate position measurements at the edge of themeasurement area.

One aspect of the present invention provides an apparatus fortransferring signals between first and second devices which alleviatesone or more of the above problems.

According to this aspect, the present invention provides an apparatushaving a planar working area and at least one winding whose effectivemagnetic axis is non-orthogonal to said planar working area. The windingmay be used as an excitation coil to energise a remote electromagneticdevice (such as a stylus) and/or it may be used to receive signalstransmitted by the remote electromagnetic device.

According to another aspect, the present invention provides a positionsensor having first and second relatively movable members, the firstmember having a plurality of windings arrayed over the x-y planarworking area and each winding comprising at least two loops wound inopposite senses; the second member comprising an electromagnetic deviceoperable to interact with the windings to generate signals that varywith the relative position of the first and second members; and whereinthe loops of said plurality of windings are arranged in at least tworows and at least two columns with the loops in the same column beingwound in the same sense and with adjacent loops in the same row beingwound in alternative sense. Such windings may be used to either transmitsignals to or to receive signals from the electromagnetic device.

According to a further aspect, the present invention provides a circuitboard having at least two conductor loops one of which is provided in acentral portion and the other of which is provided in a peripheralportion which is flexible relative to the central portion.

Exemplary embodiments of the present invention will now be describedwith reference to the accompanying drawings in which:

FIG. 1 is a perspective view showing a mobile telephone having a liquidcrystal display and an x-y digitising system located behind the displaywhich can sense the position and status of a resonant stylus;

FIG. 2 a is a schematic functional block diagram illustrating theexcitation and processing electronics of the x-y digitising system andillustrating the magnetic coupling between excitation windings of thedigitising system and the resonant stylus and the magnetic couplingbetween the resonant stylus and four sensor windings which form part ofthe digitising system;

FIG. 2 b is a timing plot illustrating the form of various signalswithin the x-y digitising system shown in FIG. 2 a during an excitationand receive cycle;

FIG. 3 a is an exploded perspective view of the resonant stylus shown inFIG. 1;

FIG. 3 b is a cross-sectional view of the resonant stylus shown in FIG.1;

FIG. 4 a is a schematic diagram illustrating the way in which theexcitation windings shown in FIG. 2 a are wound around a printed circuitboard forming part of the x-y digitising system;

FIG. 4 b schematically illustrates the arrangement of the excitationwindings wound around the printed circuit board viewed along the longside of the printed circuit board;

FIG. 4 c schematically illustrates a cross-section of the excitationwindings and the printed circuit board shown in FIG. 4 a;

FIG. 5 a schematically illustrates the magnetic field created by passinga current through one of the excitation windings shown in FIG. 4 c;

FIG. 5 b schematically illustrates the form of the magnetic fieldgenerated by passing a current through the other excitation windingsshown in FIG. 4 c;

FIG. 5 c is a plot illustrating the way in which the magnetic couplingbetween the resonant stylus and the two excitation windings varies withposition of the stylus in an x-direction of the digitising system;

FIG. 6 schematically illustrates the way in which notches may be madealong the edge of the printed circuit board to facilitate the accuratewinding of the excitation coils around the printed circuit board;

FIG. 7 a schematically illustrates the form of a folded printed circuitboard carrying the four sensor windings illustrated in FIG. 2 a;

FIG. 7 b illustrates the outline of the flexible printed circuit boardand the fold lines of the circuit board shown in FIG. 7 a;

FIG. 7 c schematically illustrates the arrangement of the four sensorwindings on the flexible printed circuit board shown in FIG. 7 b;

FIG. 7 d shows the conductor tracks which are formed on one layer of theflexible printed circuit board together with the correspondingconnection pads;

FIG. 7 e illustrates the conductor tracks formed on a second layer ofthe flexible printed circuit board which, together with the tracks onthe first layer, create the four sensor windings illustrated in FIG. 2a;

FIG. 8 a schematically illustrates the form of the magnetic fieldgenerated by the passing current through a sin x sensor winding;

FIG. 8 b is a plot illustrating the way in which the magneticsensitivity of the sin x sensor winding varies with the x position ofthe stylus;

FIG. 9 a schematically illustrates the magnetic field generated bypassing a current through a cos x sensor winding;

FIG. 9 b is a plot illustrating the way in which the magneticsensitivity of the cos x sensor winding varies with the x position ofthe stylus;

FIG. 10 schematically illustrates the form of four sensor windings whichmay be used in combination with or instead of the sensor windings formedon the flexible printed circuit board shown in FIG. 7;

FIG. 11 a schematically illustrates the form of a further alternativeset of sensor windings which may be used instead of or in combinationwith the sensor windings shown in FIG. 7;

FIG. 11 b illustrates the pattern of conductors forming two of thewindings shown in FIG. 11 a;

FIG. 11 c illustrates the pattern of conductors forming the other twowindings shown in FIG. 11 a;

FIG. 12 schematically illustrates a rotatable drum on which theexcitation coils can be wound onto the printed circuit board;

FIG. 13 a schematically illustrates the form of an alternative set ofexcitation coils arrayed over a printed circuit board of an x-ydigitising system;

FIG. 13 b shows a section of a printed circuit board illustrating afurther alternative set of excitation windings wound on the printedcircuit board with their axis in different directions but lying parallelto the plane of the circuit board;

FIG. 14 a schematically illustrates the form of a further alternativeexcitation winding arrangement formed by conductor tracks printed on theprinted circuit board;

FIG. 14 b illustrates the physical arrangement of the printed circuitboard and a flexible piece of magnetically permeable material whichpasses through two slots in the printed circuit board around which theprinted coils are wound;

FIG. 14 c schematically illustrates the arrangement shown in FIG. 14 bviewed from beneath the printed circuit board;

FIG. 15 a is a block diagram illustrating a system model which is usedto derive the position and status information of the resonant stylusfrom the signal measurements received from the sensor windings; and

FIG. 15 b schematically illustrates a preferred form of the system modelshown in FIG. 15 a.

OVERVIEW OF DIGITISING SYSTEM

FIG. 1 shows a handheld battery-powered mobile cellular telephone 1which employs an x-y digitising system (not shown) that is associatedwith a liquid crystal display (LCD) 3 of the telephone 1. The x-ydigitising system is operable to detect the presence and x-y position ofa resonant stylus 5 relative to the LCD 3. The position of the signalsoutput from the digitising system are used by the mobile telephone tocontrol information that is displayed on the LCD 3 and to control theoperating function of the telephone 1.

FIG. 2 a schematically illustrates a functional block diagram of thedigitising system's processing electronics and FIG. 2 b illustrates someof the signals in the digitising system during an excitation and receivecycle. FIG. 2 a also illustrates the way in which excitation windingsand the sensor windings interact with the resonant stylus 5. Inparticular, FIG. 2 a schematically shows two excitation windings 29-aand 29-b, two x-sensor windings 31 and 33 for sensing x position and twoy-sensor windings 35 and 37 for sensing y position. As illustrated bythe arrows 39, these windings are operable, in use, to coupleelectromagnetically with a resonant circuit 41 (comprising a capacitor43 and an inductor coil 45) in the resonant stylus 5.

In operation, an excitation current is applied to one of the excitationwindings 29 through an excitation driver 51 and switch 56. In thisembodiment, the excitation current comprises a sequence of positive andnegative pulses having a fundamental frequency component (F₀) ofapproximately 100 kHz, which is approximately the resonant frequency ofthe resonant circuit 41. This excitation signal is generated by avariable frequency generator 53 which generates an appropriateexcitation voltage which is applied to the excitation driver 51 througha switch 55. In this embodiment, the frequency of the excitation voltagegenerated by the generator 53 is set by an excitation/receive frequencycontrol circuit 57 which forms part of a digital processing and signalgeneration unit 59. As those skilled in the art will appreciate, byusing such a variable frequency generator 53, the digitising system canbe reconfigured to operate with a stylus having a different resonantfrequency.

The excitation current flowing in the excitation windings 29 generates acorresponding electromagnetic field which couples, as indicated by thearrows 39-0 and 39-1, with the resonant circuit 41 and causes it toresonate. When the resonator 41 is resonating, it generates its ownelectromagnetic field which couples, as represented by the arrows 39-2,39-3, 39-4 and 39-5, with the sensor windings 31, 33, 35 and 37respectively. As will be explained in more detail below, the sensorwindings 31, 33, 35 and 37 are designed so that the coupling betweenthem and the resonant stylus varies with the x or y position of thestylus 5. Therefore, the signals received in the sensor windings willvary with the position of the resonator 41 relative to the respectivesensor winding. Consequently, by suitable processing of the signalsreceived in the sensor windings, the x-y position of the resonator 41,and hence of the resonant stylus 5, can be determined relative to thesensor windings.

In this embodiment, the excitation current is not continuously appliedto the excitation winding 29. Instead, bursts of the excitation currentare applied, with the application of the excitation bursts beingcontrolled by opening and closing the switch 55. As shown in FIG. 2 a,this is controlled by an excitation gate controller 61 which forms partof the digital processing and signal generation unit 59. An excitationselector 62 (also forming part of the digital processing and signalgeneration unit 59) controls the position of the switch 56 to controlwhich of the excitation windings 29 is to be energised. In thisembodiment, in order to reduce the effect of any breakthrough from theexcitation winding 29 to the sensor windings, the signals induced in thesensor windings are only detected between the bursts of the excitationcurrent. This is achieved by controlling the positions of switches 63and 65 with the receive gate controller 67 which forms part of thedigital processing and signal generation unit 59. This mode of operationis referred to as pulse echo and works because the resonator 41continues to resonate after the burst of excitation current has ended.This mode of operation also minimises power consumption of thedigitiser.

FIG. 2 b shows the excitation gate signal 30-1 applied to the switch 55;the excitation voltage 30-2 applied to one of the excitation windings29; the receive gate signal 30-3 applied to the switches 63 and 65 and atypical voltage 30-4 induced in one of the sensor windings. In thisillustration, sixteen excitation cycles (counting the start and endpulses as halves) are applied to the excitation winding 29 whichenergises the resonator 41 in the stylus 5 which in turn induces asignal such as 30-4 in each of the sensor windings. In this embodiment,the sensor windings 31, 33, 35 and 37 used in this embodiment and are inspatial phase quadrature relative to each other and are arranged to havesinusoidal coupling factors with the resonant stylus 5. In view of thesinusoidal coupling and quadrature arrangement of the sensor windings,the four signals induced in the four sensor windings from the resonantcircuit 41 can be approximated by: $\begin{matrix}{{E_{31}(t)} = {A\quad{\mathbb{e}}^{{- t}/\tau}{\sin\left\lbrack \frac{2\quad\pi\quad x}{L_{x}} \right\rbrack}{\cos\left\lbrack {{2\quad\pi\quad F_{o}t} + \varphi} \right\rbrack}}} & (1) \\{{E_{33}(t)} = {A\quad{\mathbb{e}}^{{- t}/\tau}{\cos\left\lbrack \frac{2\quad\pi\quad x}{L_{x}} \right\rbrack}{\cos\left\lbrack {{2\quad\pi\quad F_{o}t} + \varphi} \right\rbrack}}} & (2) \\{{E_{35}(t)} = {A\quad{\mathbb{e}}^{{- t}/\tau}{\sin\left\lbrack \frac{2\quad\pi\quad y}{L_{y}} \right\rbrack}{\cos\left\lbrack {{2\quad\pi\quad F_{o}t} + \varphi} \right\rbrack}}} & (3) \\{{E_{37}(t)} = {A\quad{\mathbb{e}}^{{- t}/\tau}{\cos\left\lbrack \frac{2\quad\pi\quad y}{L_{y}} \right\rbrack}{\cos\left\lbrack {{2\quad\pi\quad F_{o}t} + \varphi} \right\rbrack}}} & (4)\end{matrix}$where A is a coupling coefficient which depends upon, among otherthings, the distance of the stylus 5 from the windings and the number ofturns in the sensor windings; x is the x-position of the resonant stylusrelative to the sensor windings; y is the y-position of the resonantstylus relative to the sensor windings; L_(x) is a spatial wavelength ofthe sensor windings in the x-direction; Ly is a spatial wavelength ofthe sensor windings in the y-direction; e^(−t/τ) is the exponentialdecay of the resonator signal after the burst of excitation signal hasended, with τ being a resonator constant which depends upon, among otherthings, the quality factor of the resonant circuit 41; and φ is anelectrical phase shift caused by a difference between the fundamentalfrequency of the excitation current and the resonant frequency of theresonator 41. In this embodiment, the resonant stylus 5 is designed sothat its resonant frequency changes with the pressure applied to the tipof the stylus. This change in frequency causes a change in the phaseshift φ.

Therefore, both the x-y position information of the resonant stylus 5and the phase shift φ can be determined from the signals induced in thesensor windings by suitable demodulation and processing. As shown inFIG. 2 a, this demodulation is achieved by mixing the received signalswith the excitation voltage generated by the variable frequencygenerator 53 in the mixers 69-1 to 69-8. In this embodiment, an in-phasecomponent 30-5 and a quadrature phase component 30-6 (shown in FIG. 2 b)of the excitation signal are mixed with the signal induced in each ofthe sensor windings. This generates an in phase (I) component 30-7 and aquadrature phase (Q) component 30-8 of each of the demodulated signals.In this embodiment, the in phase components 30-7 of the demodulatedsignals from all the sensor windings are used to determine the positioninformation and the in phase and quadrature phase components of thedemodulated signals are used to determine the electrical phase shift(i.e. φ). As shown in FIG. 2 a, the output from each mixer 69 is inputto a respective integrator 71-1 to 71-8 which, after being reset,integrates the output from the mixer over a time period which is amultiple of 1/F₀ (in order to remove the effect of the time varyingcomponents output by the mixer). In this embodiment, the integrationtime is controlled by using the receive gate signal 30-3 (which in theillustration allows for the integration to be performed over sixteenexcitation periods or cycles). The following equations approximate theoutputs from the integrators 71-1 to 71-4: $\begin{matrix}{{{sin\_ x}{\_ I}} = {A_{1}{\sin\left\lbrack \frac{2\quad\pi\quad x}{L_{x}} \right\rbrack}\cos\quad\varphi}} & (5) \\{{{sin\_ x}{\_ Q}} = {A_{1}{\sin\left\lbrack \frac{2\quad\pi\quad x}{L_{x}} \right\rbrack}\sin\quad\varphi}} & (6) \\{{{cos\_ x}{\_ I}} = {A_{1}{\cos\left\lbrack \frac{2\quad\pi\quad x}{L_{x}} \right\rbrack}\cos\quad\varphi}} & (7) \\{{{cos\_ x}{\_ Q}} = {A_{1}{\cos\left\lbrack \frac{2\quad\pi\quad x}{L_{x}} \right\rbrack}\sin\quad\varphi}} & (8)\end{matrix}$where A₁ is a constant which varies with, among other things, theconstant A, the resonator τ and the integration period. Similar signalsare obtained from integrators 71-5 to 71-8, except these vary with they-position rather than with the x-position. FIG. 2 b also illustratesthe output voltage 30-9 from one of the in-phase integrators and theoutput voltage 30-10 from one of the quadrature phase integrators.

As shown in FIG. 2 a, the outputs from the integrators 71 are input toan analogue-to-digital converter 73 which converts the outputs intodigital values which are input to the A to D interface unit 75 in thedigital processing and signal generation unit 59. The digital processingand signal generation unit 59 then performs an arc tangent function(atan2) on the sin_x_I signal and the cos_x_I signal to determine thex-position of the resonant stylus 5 and similarly performs an arctangent function on the sin_y_I signal and the cos_y_I signal todetermine the y-position of the resonant stylus 5. The digitalprocessing and signal generation unit 59 also calculates an arc tangentfunction on the quadrature phase component and the in phase component ofthe signals from the same sensor windings, in order to determine theelectrical phase angle φ.

As shown in FIG. 2 a, the in phase and quadrature phase component forthe signal induced in each of the sensor windings is calculated. This isbecause, at certain x and y positions, the ratio of the in phase andquadrature phase components from some of the sensor windings will not bereliable. This occurs when the sin or cos position components areapproximately zero. Therefore, in this embodiment, the digitalprocessing and signal generation unit 59 determines the electrical phaseangle φ using a weighted combination of the in phase and quadraturephase signals from both the sin and cos windings, where the weightingused varies in dependence upon the determined x and y position of thestylus 5. The processing electronics then uses this electrical phaseangle measurement to determine if the tip of the stylus 5 has beenbrought down into contact with the writing surface of the telephone 1.

Returning to FIG. 2 a, after the digital processing and signalgeneration unit 59 has determined the current x-y position of theresonant stylus 5 and determined whether or not the stylus 5 has beenbrought into contact with the LCD 3, it outputs this information to thetelephone's electronics through the interface unit 77. This informationis then used by the telephone's electronics to control informationdisplayed on the LCD 3 and the telephone's mode of operation. In thisembodiment, the digital processing and signal generation unit 59 isoperable to perform the above calculations approximately 100 times persecond when the stylus is in the vicinity of the telephone. However,when the system detects that the stylus 5 is not present, it initiallyenters a standby state in which the above excitation and processing isperformed approximately 20 times per second. After a predeterminedlength of time in this standby state, the system enters a sleep state inwhich the above calculations are performed approximately 2 times persecond. Once the presence of the stylus 5 is detected again, theprocessing resumes at the 100 times per second rate.

As discussed above, the resonant stylus 5 used in the present embodimentcomprises a resonant circuit 41 which includes an inductor coil 45 and acapacitor 43. The resonant stylus 5 is also designed so that theresonant frequency of the resonant circuit 41 changes when the tip ofthe stylus 5 is brought down into contact with the writing surface ofthe digitising system. The particular structure of the resonant stylus 5used in this embodiment is shown in an exploded view in FIG. 3 a. Asshown, the stylus 5 comprises a hollow front body portion 152 and ahollow rear body portion 154 which house: the resonant circuit 41comprising the inductor coil 45 and the capacitor 43; a 2 mm diameterferrite rod 153; a plastic sleeve 155 having an inner diameter of 2.1 mmand an outer diameter of 2.2 mm; a split washer 157; a nib 159; and aspring 163. The coil 45 is manufactured from self-bonding enamelledcopper wire for low-cost by eliminating a coil former. The ends of thecoil 45 are welded to the side of a surface mount capacitor 43 to formthe resonant circuit 41. The plastic sleeve 155 having a thin wallsection (of approximately 50 microns) made from spirally wound andbonded plastic sheet fits inside the coil 45 and acts as a bearingsurface for the ferrite rod 153 and prevents the ferrite rod 153 fromrubbing against the capacitor 43 during use. The plastic sleeve 155 hasa much thinner cross-section than can be achieved with aninjection-moulded component, thereby enabling higher resonator Q-factorand hence lower system power consumption.

FIG. 3 b shows the assembled stylus 5 in cross-section. The nib 159 andthe ferrite rod 153 are slidably mounted within the stylus body andspring-biased (by spring 163) towards the front end 161 of the frontbody portion 152. The movement of the ferrite rod 153 in this forwarddirection is, however, limited by the abutment of a front face 160 a ofan enlarged head 160 of the nib 159 with a second shoulder 168 of thefront body portion 152. When pressure is applied to the nib 159 of thestylus 5 against the biasing force of the spring 163, the nib 159 andthe ferrite rod 153 move towards the rear body portion 154 until a rearface 160 b of the nib's head 160 abuts against the split washer 157. Asshown in FIG. 3 b, the ferrite rod 153 can, therefore, only move apredetermined distance (d₀) when pressure is applied to the end of thenib 159. In this embodiment, the stylus 5 is designed so that the clickdistance (d₀) is 0.35 mm. This movement of the front face 153 a of theferrite rod 153 from the front face 45 a of the coil 45 causes adecrease in the inductance of the coil 45 due to the reduced couplingbetween the ferrite rod 153 and the coil 45, which in turn gives rise toan increase in the resonant frequency of the resonant circuit 41. Theprocessing electronics can then detect when the stylus 5 is brought intocontact with the writing surface from the value of the phase angle φ,which varies with the resonant frequency of the stylus 5.

A brief description has been given above of the way in which thedigitiser system of the present embodiment determines the x-y positionand the status of the resonant stylus 5 relative to the sensor windings.The particular form of excitation and sensor windings used in thisembodiment will now be described in more detail.

Excitation Windings

FIG. 4 a is a schematic diagram illustrating a circuit board 13 aroundwhich the two excitation windings 29-a and 29-b are wound. In thisembodiment, a layer of magnetic material (not shown) is laminated to thecircuit board 13 and the excitation windings 29 are wound around boththe circuit board 13 and the layer of magnetic material. The layer ofmagnetic material provides a magnetic path for the field generated bythe excitation winding 29 so that more of the field extends to the edgeof the circuit board 13. The material used for this layer of magneticmaterial depends on the excitation frequencies to be applied to theexcitation winding 29. For frequencies in the range of 0 to 100 kHz, a50 μm to 200 μm mumetal is an optimum screen material. However, forhigher frequencies up to for example 500 kHz, spin melt ribbon ispreferably used.

As shown in FIG. 4 a, the two excitation windings 29-a and 29-b arewound in opposite directions around the printed circuit board 13. FIG. 4b illustrates the view of the assembly shown in FIG. 4 a when viewedtowards the longer edge of the printed circuit board 13 and FIG. 4 cillustrates a cross-sectional view of the arrangement shown in FIG. 4 a.

FIG. 5 a illustrates the magnetic field Mexb that is generated by theexcitation winding 29-b when a current is passed through it (ignoringthe field generated below the circuit board 13). FIG. 5 a alsoillustrates the coil 45 of the resonant stylus 5 which is designed toreceive the magnetic field Mexb to thereby power the stylus forre-radiating to the sensor windings of the x-y digitiser system. Asdiscussed above, the stylus 5 is moveable in any direction over theworking area of the x-y digitising system although FIG. 5 a onlyillustrates movement in the x-direction in view of the cross-sectionalview. FIG. 5 b illustrates the corresponding magnetic field Mexa thatthe other excitation winding 29-a generates when a current is passedthrough it (again ignoring the field generated below the circuit board13).

FIG. 5 c is a plot illustrating the way in which the magnetic coupling(Kra) between the excitation winding 29-a and the coil 45 and themagnetic coupling (Krb) between the excitation winding 29-b and the coil45 varies with the position of the coil 45 along the x-direction abovethe printed circuit board 13. As shown, each of the two plots includes apoint where the coupling factor is zero. When the stylus 5 is heldperpendicular to the circuit board 13, this point is approximately whenthe stylus 5 is directly above the respective excitation winding 29.Further, as can be seen from FIG. 5 c, there are other positions in theplots where the coupling factor is low. In particular, excitationwinding 29-a is off centre to the left and the corresponding couplingfactor (Kra) is low when the stylus 5 is to the extreme right-hand sideof the circuit board 13. Similarly, excitation winding 29-b is offcentre to the right and the magnetic coupling factor (Krb) is low whenthe stylus 5 is to the extreme left-hand side of the circuit board 13.However, in each case, the other excitation winding 29 has a relativelyhigh magnitude of coupling factor and it is therefore possible tomaintain transmitting power to the stylus 5 regardless of the x-positionof the stylus relative to the circuit board 13.

As shown in FIG. 4 a, the geometry of the excitation windings 29 aresubstantially uniform in the direction perpendicular to the x-directionand in the plane of the circuit board (i.e. in the y-direction). Themagnetic field lines illustrated in FIG. 5 are therefore relativelyuniform in the y-direction so that the system is able to power thestylus 5 for any given y-position in the vicinity of the circuit board13.

If the user tilts the stylus 5 about the y-axis, then this is broadlyequivalent to an offset in the x-position in the plot shown in FIG. 5 c.However, since the stylus 5 can be powered for any given x-position inproximity of the circuit board 13, such tilting of the stylus 5 aboutthe y-axis does not affect the powering of the stylus 5. If the usertilts the stylus 5 about the x-axis, with the tip of the stylus 5 at aconstant position, then this has the effect of: i) reducing the couplingfactor due to the coil 45 in the stylus 5 being at an angle to theexcitation magnetic field; and ii) increasing the coupling due to thecloser proximity of the stylus 5 to the circuit board 13. It has beenfound that these two effects largely cancel each other out. Therefore,although tilting the stylus 5 may alter the coupling factors between theexcitation windings 29 and the coil 45 in the stylus 5, it is stillalways possible to power the stylus 5 by selecting which excitation coil29 to power depending on the current x-y position of the stylus 5.

In this embodiment, the current position of the stylus 5 (as determinedby the processing electronics) is used to control which one of the twoexcitation windings 29 is used to power the stylus. In this embodiment,data representing the magnetic coupling plots shown in FIG. 5 c arestored within the processing electronics. The determined currentposition of the stylus 5 is then compared with this data to identifywhich one of the two excitation windings 29 has the largest magneticcoupling with the stylus 5 and this excitation coil is the one used topower the stylus 5.

As shown in FIG. 6, in this embodiment slots 201 are provided along theedge of the circuit board 13 to facilitate the accurate winding of theexcitation winding 29 around the circuit board 13. The free ends of theexcitation windings 29 are then connected to the appropriate connectionpads on the printed circuit board 13 by soldering or welding or by someother connection technology.

Unlike the previous stylus powering approaches described in the priorart discussed above, the excitation windings 29 are arranged so thattheir magnetic axes are non-orthogonal (and indeed are preferablyparallel) to the plane of the printed circuit board 13. The inventor hasfound that this arrangement of the windings 29 provides a more efficientcoupling to the coil 45 in the stylus 5 than with the previous designssuch as those described in U.S. Pat. No. 4,878,553 or WO 00/33244discussed above.

Additionally, the inventor has found that with this new geometry of theexcitation windings 29, it is possible to power the stylus 5 at fargreater distances from the perimeter of the circuit board 13 than withthe previous designs. This is because with the previous design of theexcitation windings, the magnetic field powering the stylus 5 issubstantially parallel to the circuit board 13 near to its edges, withthe result that the axis of the coil 45 in the stylus (being usuallyroughly perpendicular to the circuit board 13) is at such an angle tothe magnetic field as to reduce the coupling between that field and thestylus 5 to below acceptable levels. Further, when the component of thefield parallel to the axis of the coil 45 in the stylus 5 changes sign,the sign of the coupling changes and the place at which this occurs maynot be known. With the new geometry, the magnetic field powering thestylus 5 is roughly perpendicular to the circuit board 13 even near itsedges, and the field component parallel to the axis of the coil 45 doesnot change sign near the edge. In the present embodiment, the regions inwhich the excitation field arising from one of the excitation winding 29becomes parallel to the board, are among those in which the otherexcitation winding is used to energise the stylus 5.

Additionally, in this embodiment, since the excitation windings 29 arenot formed from printed conductors but from wire wound coils, theexcitation windings can be formed from a much greater mass of copper.This additional mass of copper yields lower excitation winding lossesfor a given excitation field, thereby minimising power loss and hencemaximising battery life.

The excitation magnetic field generated by the excitation windings 29used in this embodiment will also have a smaller coupling to anyconductive metal bezel placed around the perimeter of the circuit board13. This is because most of the excitation magnetic field lines shown inFIGS. 5 a and 5 b do not link with the loop formed by the bezel. Most ofthe field lines both emerge and re-enter the magnetic material laminatedto the circuit board 13 inside the bezel and most of those that do not,both emerge and re-enter this magnetic material outside the bezel loop.

The above described excitation windings may be used with any knownsensor windings such as those described in WO 00/33244 and will bemounted below the display 3 of the telephone 1. However, the preferredform of the sensor windings used in this embodiment will now bedescribed.

Sensor Windings

In this embodiment, the sensor windings are formed on a folded printedcircuit board 131 which is illustrated in FIG. 7 a. As shown, thecircuit board 131 is folded so as to fit in front of and around the sideof the liquid crystal display 3. The circuit board 131 also includes atransparent window 213 so that the active area of the display 3 is notcovered. The folded circuit board 131 therefore includes portions 214that lie in a plane parallel to the plane of the LCD 3 and portions 215which lie in planes perpendicular to the plane of the LCD 3. The printedcircuit board 131 also includes a folded connector portion 217 whichcarries the connection pads for the sensor windings and is used toconnect the sensor windings to the processing electronics.

FIG. 7 b illustrates the outline of the flexible printed circuit board131 used in this embodiment, showing the fold lines of the circuit boardas dash lines 219. FIG. 7 c schematically illustrates the position ofthe four sensor windings 31,33,35 and 37, each of which is split intotwo portions a,b located on opposite sides of the rectangular window213. As shown, the sin x sensor winding 31 is formed by two portions31-a and 31-b which both include two turns of conductor and which areconnected in series so that the turns of conductor in portion 31-a arewound in the opposite direction to the turns of conductor in portion31-b. Similarly, the sin y sensor winding 35 is formed by two portions35-a, 35-b which both include two turns of conductor and which areconnected in series so that the conductor turns of the first portion35-a are wound in the opposite direction to the conductor turns in thesecond portion 35-b. As shown in FIG. 7 c, since the sin x and sin ysensor windings are formed on the inside of the fold line 219, thesesensor windings will lie, in use, in a plane that is substantiallyparallel to the plane of the LCD display 3.

The cos x sensor winding 33 is also arranged in two portions 33-a, 33-bwhich are both formed by two turns of conductor. However, these twoportions are connected in series so that the turns of conductor in thefirst portion 33-a are wound in the same direction as the turns ofconductor in the second portion 33-b. Similarly, the cos y sensorwinding 37 is also formed by a first portion 37-a and a second portion37-b which each include two turns of conductor which are connected inseries so that they are wound in the same sense. However, since the cosx and cos y sensor windings 33 and 37 are provided on the outside of thefold line 219, in use, these windings will lie in planes that aresubstantially perpendicular to the plane of the LCD 3. The effectivemagnetic axis of these windings will therefore lie substantiallyparallel to the plane of the LCD 3. In this embodiment, the conductortracks used to generate the sensor windings are formed as printedconductors on two layers of the flexible circuit board 131 and connectedtogether where appropriate at via holes. The particular arrangement ofthe conductor tracks on these two layers that are used in thisembodiment are shown in FIGS. 7 d and 7 e, which also show theconnection pads on the connector portion 217 for connecting the sensorwindings to the processing electronics.

As discussed in the introduction, in this embodiment the sensor windingsare arranged so that the magnetic coupling between the resonant stylus 5and the sensor windings varies with the x or y position in a sinusoidalmanner. To illustrate that this is the case, consideration should begiven to the magnetic field generated by each of the sensor windingswhen a current is applied to it, since (due to the reciprocal nature ofelectromagnetic coupling) this will also define the sensitivity of thesensor winding to magnetic field generated by the resonator 41 in thestylus 5. FIG. 8 a illustrates the magnetic field lines generated by thesin x sensor winding 31 when a current is applied to it.

For clarity, only one turn of each of the two coil portions 31-a and31-b is shown. In use, the coil 45 within the stylus 5 will always belocated above the sensor winding 31 and will move, for example, alongthe dotted line 231. Therefore, considering the vertical component ofthe magnetic field along the dotted line 231 yields the effectivemagnetic sensitivity of the sip x sensor winding 31 to a vertically-heldstylus 5.

As can be seen by examining the magnetic field lines in FIG. 8 a, thevertical component of the magnetic field will be at its most positivevalue in the centre of the loop defined by conductors 31-a 1 and 31-a 2and will have a similar but negative peak value in the centre of theloop defined by conductors 31-b 1 and 31-b 2. As those skilled in theart will appreciate, the peak is negative over the right-hand portion31-b because the turns of this portion are wound in the opposite senseto the turns of the first portion 31-a. The corresponding sensitivityplot for the sin x sensor winding is, therefore, shown by the plot S₃₁^(v) shown in FIG. 8 b. As shown, the sensitivity function varies in anapproximate sinusoidal manner, with the pitch (L_(x)) of the sinusoidalvariation being approximately twice the width of the circuit board 131in the x direction (which is defined between the ordinate axis and thevertical line 237).

FIG. 9 a illustrates the magnetic field generated by the cos x sensorwinding 33 when current is applied to it. As shown, the magnetic axis ofthe two portions 33 a, 33 b lie in a plane parallel to the plane 235 ofthe LCD display 3. Considering the vertical component of the magneticfield experienced by the stylus 5 when moving in the x direction alongthe dotted path 231, it is close to zero directly above each of the twoportions 33 a, 33 b and peaks between these sensor winding portions. Thecorresponding vertical magnetic sensitivity function S₃₃ ^(v) istherefore shown in FIG. 9 b. As shown, this sensitivity function varieswith the x position of the stylus 5 in an approximate sinusoidal manner,with the period of the variation corresponding to that of the sinusoidalvariation of the sin x winding 31 but shifted along the x-direction sothat the sinusoidal variations are in spatial phase quadrature.

Additionally, and as shown in FIGS. 8 b and 9 b, the sensitivity plotsS₃₁ ^(H) and S₃₃ ^(H) of the sin x and cos x sensor windings to thehorizontal component of the magnetic field generated by the resonantstylus 5, also vary in an approximate sinusoidal (and quadrature)manner. This ensures that whatever angle the stylus 5 is held at, thecouplings to the sin and cos sensor windings will be in phasequadrature.

As can be seen by comparing FIGS. 8 b and 9 b, the sensitivity functionslose their sinusoidal characteristic in the centre of the circuit board131. However, because the processing electronics performs a ratiometriccalculation of the signals from these two windings, these irregularitiesin the sensitivity functions cancel each other out.

With regard to the sin y and cos y sensor windings 35, 37, these willhave similar sensitivity functions but which vary with the y position ofthe stylus 5 relative to the LCD 3.

The new design of the sensor windings has a number of advantages overthe prior art windings such as those described in WO 00/33244. Inparticular, since the peaks in the sin winding sensitivity functionsoccur at the edge of the circuit board 131 and since the cos windingsensitivity functions are zero at the edge of the circuit board 131, thesensitivity functions for these sensor windings maintain theirsinusoidal characteristic well beyond the edge of the circuit board 131.Therefore, tilt correction algorithms which rely on this sinusoidalvariation beyond the edge of the circuit board 131 can correct for tilteven when the stylus is located at the edge of the circuit board 131.

Another advantage of the sensor winding design used in this embodimentis that the sensor windings are all located at the edge of the circuitboard and, as a result, the signal levels do not fall off when thestylus 5 approaches the edge of the circuit board 131 which therebyimproves power consumption and accuracy. Further, since the sensorwindings do not occupy the active area of the LCD display 3, the circuitboard 131 can be provided on top of the display (so that the display canbe seen through the rectangular window 213). In this way, the sensorwindings will be positioned closer to the actual writing surface of thetelephone which improves signal level and reduces coupling to othersensitive electronics in the mobile telephone. This arrangement may alsoallow the elimination of the magnetic shield that is usually placedbetween the circuit board 131 and the mobile telephone's electronics,thereby saving cost and thickness.

The sensor windings described above may be used together with aconventional set of sensor windings underneath the display 3, to improvesignal levels and accuracy especially when the stylus 5 is in the centreof the display 3. These additional sensor windings may be formed on thecircuit board 13 around which the excitation windings 29 are wound. Anexample of a conventional set of x-y sensor windings which can be usedis shown in FIG. 10. As shown, the set of sensor windings includes a sinx sensor winding 265, a cos x sensor winding 263, a sin y sensor winding261 and a cos y sensor winding 267, which are all formed on the printedcircuit board 13. These additional sensor windings may be connectedseparately to the processing electronics or they may be connected inseries with the corresponding sensor windings on the flexible circuitboard 131.

A problem with the conventional set of sensor windings shown in FIG. 10is that it includes many conductors near the edge of the circuit board13 and, in particular, at the corners thereof. This problem is increasedfurther if each of the sensor windings 261,263,265,267 includes multipleturns of conductor. The position of the conductors at the corners maytherefore be limited by the manufacturing technique used to manufacturethe circuit board 13. For example, if the conductors are implementedusing 0.1 mm wide conductor tracks separated by 0.1 mm gaps on aconventional printed circuit board, then there can be a maximum of only5 tracks per millimetre around the edge on each layer of the printedcircuit board. Since the position of the conductors is critical to theaccuracy of the sensor windings, this problem is particularly importantnear the corners. Whilst this problem can be overcome by increasing thesize of the circuit board, this is impractical in space criticalapplications such as in the present mobile telephone device 1.

This problem with the conventional sensor winding layout shown in FIG.10 can also be overcome by merging selected x and y sensor coilstogether and then recovering the information from the merged coils inthe position processing electronics. FIG. 11 a schematically illustratesa new set of sensor windings which may be used underneath the LCD 3 ofthe mobile telephone 1. As shown in FIG. 11 a, four separate windings271,273,275,277 are provided arrayed over the circuit board 13. However,as can be seen by comparing FIG. 11 a with FIG. 10, only two conductorsare required at the edges and corners of the circuit board FIG. 11 billustrates more clearly the form of the sensor windings 271 and 275 andFIG. 11 c illustrates the form of the sensor windings 273 and 277. Ascan be seen from these Figures, each of the windings includes loopswound in opposite sense (as represented by the plus and minus symbolswithin each of the loops). Additionally, the loops of the windings arearranged in at least two rows and two columns on the circuit board 13with loops in the same column having the same polarity and with adjacentloops in the same row having alternating polarities.

The processing electronics can then recover the signal which varies withsin x by adding the signal from sensor winding 271 to the signal fromsensor winding 275. Similarly, the processing electronics can recoverthe signal which varies with sin y by subtracting the signal from sensorwinding 271 from the signal from sensor winding 275. Similarly, theprocessing electronics can recover the cos x sensor signal by adding thesignals received from sensor windings 273 and 277 and can recover thecos y sensor signal by subtracting the signal received from sensorwinding 273 from the signal received from sensor winding 277.

As can be seen from FIG. 11, with the new layout of sensor windings,there are half as many sensing conductors at the corners than with theconventional layout illustrated in FIG. 10. This means that the exactlocation of each conductor track can be chosen more freely in order tooptimise accuracy, or the number of turns for each sensor winding may beincreased in order to improve the signal to noise ratio.

Modifications and Alternative Embodiments

The embodiment described above describes a mobile telephone having anx-y digitising system for sensing the position of a user controlledresonant stylus. A novel arrangement of the excitation winding used toenergise the resonant stylus was described together with a novelarrangement of sensor windings formed on a flexible printed circuitboard. A further novel set of sensor windings was also described whichcould be used in combination with the sensor windings on the foldedcircuit board. As those skilled in the art will appreciate, it is notessential to provide an x-y digitising system having all of these novelcomponents. For example, the novel excitation windings may be used incombination with other types of sensor windings such as the loop coilsdescribed in U.S. Pat. No. 4,878,553. Similarly, the novel sensorwindings may be used together with a conventional excitation windingwhose magnetic axis is orthogonal to the plane of the x-y measurementarea. Further still, the two novel sensor winding designs describedabove do not have to be used together; each may be used separately ifdesired.

In the above embodiment, the novel excitation coil was used to energisea resonant stylus which in turn re-radiated a signal for reception bythe sensor windings. As those skilled in the art will appreciate, theexcitation winding(s) described above may be used in other applicationssimply to power or to transfer information to a remote electromagneticdevice which operates above the x-y working area. For example, theremote device may detect the EMFs induced in a coil thereof by the twoexcitation windings, and use the relative amplitudes of these EMFs todetermine its position relative to the windings. This positioninformation can then be stored or used by the device or relayed toanother device through an appropriate transmission channel (RF, optic,acoustic etc). Further, if the remote electromagnetic device is torespond when energised, this may be limited to transmitting a statussignal which is independent of the position of the remote devicerelative to the x-y working area. Further still, the same excitationwinding may be used to both transmit signals to the remoteelectromagnetic device and to receive signals from the remoteelectromagnetic device.

In a similar manner, the novel sensing windings described above may beused in systems that do not require an excitation winding. For example,when a powered stylus is used (e.g. a battery powered stylus), there isno need for a separate excitation winding for energising the stylus. Insuch an embodiment and in the previous embodiments, the input impedanceof the processing electronics may be made high so that very littlecurrent flows in the sensor windings and the electronics detect thevoltages induced in the windings.

Additionally, as those skilled in the art will appreciate, because ofthe general reciprocal nature of electromagnetic coupling, it ispossible to reverse the operation of the above described sensor andexcitation windings. In particular, the resonator may be energised byapplying excitation current to the above-described sensor windings andby sensing the signals induced in the above-described excitationwindings.

In the above embodiment, the processing electronics controlled which ofthe two excitation windings were energised depending on the currentposition of the stylus. In an alternative embodiment, the processor mayregularly switch power between the two excitation coils. If necessary,the stylus may include an energy reservoir so that if one of theexcitation coils does not couple with the stylus the energy stored inthe energy reservoir may be used to power the stylus for the period oftime that the current is applied to that excitation coil. This approachmay be used in conjunction with the technique described in the mainembodiment, for example when the position of the stylus in unknown suchas at the outset of position sensing.

In the main embodiment described above the excitation circuitry appliedcurrent to one of the two excitation windings. Alternatively, theexcitation electronics may be arranged to apply current to bothexcitation windings simultaneously. In this case, however, the phase ofthe two excitation signals applied to the excitation windings willdepend on the current position of the stylus (to account for theopposite winding directions of the two windings). In particular, if thestylus is to the left of the winding 29-a or to the right of winding29-b then the two excitation signals should be 180° out of phase witheach other, but when the stylus is located between the two windings, thetwo excitation signals should be in phase with each other. If the twoexcitation windings are moved closer to the edge of the circuit board,then the same phase of excitation current may be applied to the twowindings.

In the above embodiments, the excitation windings were wound along thelong dimension of the circuit board. Alternatively, the excitationwindings may be wound around the shorter dimension of the circuit board.This has the benefit of shorter wire length and hence lower resistance.Losses in the excitation windings are therefore lower for a givencurrent and hence strength of magnetic field. However, this magneticfiled exists over a smaller fraction of the circuit board than when thewindings are wound along the longer dimension, and it may be necessaryto increase the number of excitation windings to enable the wholeworking area to be covered for a given minimum level of power to betransferred to the resonator.

In the above embodiments, two excitation windings were wound around thecircuit board. As those skilled in the art will appreciate, it is notessential to use two excitation windings. The number of excitationwindings used is a compromise between, on the one side complexity ofcircuit board manufacture, complexity of drive electronics andcomplexity of processing algorithms and on the other side efficiency.

In the above embodiment, the excitation windings were wound around boththe sensor printed circuit board and a layer of magnetic screeningmaterial laminated to the base of the circuit board. As those skilled inthe art will appreciate, it is not essential to have such screeningmaterial laminated to the circuit board. Further, if screening materialis provided, then the excitation windings may be wound around thescreening material alone. The selection may be made depending on theease of manufacture and any need to minimise coupling between theexcitation windings and the sensor windings.

In embodiments where electronic components are provided under theexcitation windings, a conductive screen and/or a magnetic screen may beprovided between the excitation windings and these electroniccomponents, in order to minimise interference between the two systems. Aspacer may be required between these screens and the excitation windingin order to minimise any impact they may have on power efficiency.

In the above embodiment, the excitation winding was wound around theprinted circuit board so that the magnetic axis of the excitationwinding is substantially parallel to the plane of the circuit board. Thesame result can be achieved by laying the excitation windings asconductors on two printed circuit boards with the screening materiallaminated between the two circuit boards and in which the conductors areconnected through the circuit boards and through the screening materialwhere appropriate to form continuous loops whose axes lie parallel tothe circuit boards.

In the above embodiment, the magnetic coupling between the excitationwindings and the resonator varied with the position of the resonatoralong the x axis. It is therefore possible to provide a winding aroundthe perimeter of the circuit board, to measure the resonator signalafter it has been energised by each of the excitation windings in turnand to use these measured signals to determine a coarse measurement ofthe position of the stylus.

In the above embodiment, the excitation winding was accurately woundaround the periphery of the circuit board by providing slots along theedge of the circuit board. Alternatively, the windings may be builtseparately, for example by winding self bonding wire onto a cylindricalformer whose circumference matches the final length of winding required(twice the height dimension of the circuit board). The resultingself-bonding ribbon would then be flattened and then bonded to thecircuit board in the appropriate position. Alternatively, the excitationwindings may be formed by winding the wire directly onto the circuitboard or the screen, by feeding wire onto the circuit board or screen asit is rotated about the x-axis. However, this technique suffers frompoor wire position control as the wire is placed along each long edge.

As a further alternative, and as shown in FIG. 12, the magnetic screen19 and the printed circuit board 13 may be placed on opposite sides of arotatable drum 205 and held in place with double-sided adhesive tape,which serves to fix the magnetic screen 19 and the circuit board 13 tothe drum 205 and also acts to provide a winding surface for attaching awire. The wire can then be fed under tension onto the drum in aconventional winding process. When complete, the coil ends areterminated to the printed circuit board 13 and the screen 19 and printedcircuit board 13 are brought back flat, in contact with each other andthen laminated with glue. Alternatively still, the system described withreference to FIG. 12 can be modified further to include only themagnetic screen 19 or only the circuit board 13. In this case, anadditional material would have to be provided on the other side of thedrum 205 which would be designed to release the adhesive tape oncompletion of the winding process.

In the above embodiment, the two excitation windings were wound alongthe entire length of the printed circuit board. As those skilled in theart will appreciate, this is not essential. FIG. 13 a illustrates anembodiment where two sets of excitation windings are wound in twocolumns along the length of the circuit board 13. The windings of thefirst set 29-a 1 to 29-a 7 are provided in the first column and thewindings in the second set 29-b 1 to 29-b 7 are provided in the secondcolumn. Each set of windings would then operate in a similar manner tothe windings used in the first embodiment. In this alternativearrangement, the windings in each column may be connected in series orin parallel and each set offers the advantage of reduced wire length fora given magnetic field strength due to the concentrating effect of themagnetic material which improves the efficiency further. Additionally,the individual coils in each set may be powered individually so thatonly small sections of the sensing area are powered at any time, therebyfurther improving efficiency but at the expense of complexity. Thechoice of excitation coil would then be based on the current x and ypositions. A coarse indication of y position may be obtained throughinterpolation of the signal strength detected from powering the coils inone of the columns. Additionally, the individual excitation windingsshown in FIG. 13 a may be implemented using planar self-bonded coil asused in RF-ID tags. In this case, the magnetic screen would have to beslit into overlapping parts to enable these discrete coils to be placedin position. The advantage of RF-ID tag coils is ease of manufacture andsmall thickness.

As a further alternative, the edge of the circuit board 13 may becastellated in the manner illustrated in FIG. 13 b, which allows thewinding of coils 29-y 1 to 29-y 4 with axes extending in the y directionand coils 29-x 1 to 29-x 3 with axes extending in the x direction, whichcan lead to further improvements in efficiency. Further, since thesecoils are relatively small (compared to the dimensions of the circuitboard), they may be better suited as sensor coils and will increaseaccuracy in position sensing.

In the above embodiment, the excitation winding used was wound in planesthat were orthogonal to the plane of the printed circuit board.Alternatively, as illustrated in FIG. 14, the excitation winding may beformed by conductors printed in the plane of the circuit board. In thiscase, a small flexible bridge piece of permeable material 281 (made forexample out of spin melt ribbon) would be passed through two slots 283and 285 in the printed circuit board 13 which have the printed coilsforming the excitation winding 29 wound round them. FIG. 14 a is aplanar view of such an embodiment and FIG. 14 b is a cross sectionalview showing how the flexible magnetic bridge 281 attaches to the shieldmaterial 19 underneath the circuit board 13 and passes through the slots283 and 285. FIG. 14 c is a view of the circuit board 13 from beneaththe screening material 19. In this case, because of the flexiblemagnetic material 281, the effective magnetic axis of the excitationwinding 29 is still substantially parallel to the plane of the printedcircuit board 13.

In the above embodiment, the excitation windings described above wereused in a mobile telephone device. As those skilled in the art willappreciate, the above novel windings may be used in other applications.For example, when the excitation windings are used with a tablet PC, themagnetic field which is emitted by the excitation windings can be usedto inductively power a wireless mouse which operates to the side of thetablet PC, in addition to or instead of powering the stylus. Such anembodiment would be especially useful where a tablet PC is a convertibletype where the mouse could be used in either mode of operation (as amouse or a stylus). In such an embodiment, it may be necessary to windat least one excitation coil perpendicular to the others in the plane ofthe circuit board, to enable the mouse to be powered in all positionsaround the perimeter of the tablet PC.

In an embodiment where the sensor windings mounted on the abovedescribed flexible printed circuit board are used with a conventionaltype of excitation winding (wound in the plane of the circuit board),this excitation winding may be mounted on a separate rigid printedcircuit board also having a transparent window and then laminated on topof the central region 215 of the flexible circuit board. In this case,the combined circuit board may be manufactured by using a conventionalrigid-flex manufacturing process. Additionally, where manually operableswitches are also provided, the connection for these switches may alsobe mounted on the same circuit board in order to minimise the number ofseparate printed circuit boards and connections required. A groundedconductor layer may also be added to the top of the rigid circuit boardcarrying the excitation winding and the connection tracks for theswitches. This layer can be slit so that a continuous loop does notexist around the display window which would otherwise act as a shortedturn for the excitation winding which would reduce efficiency due toeddy current losses in this conductive loop. A capacitor may also beconnected across this slit so that it acts to suppress electromagneticfields with much higher frequency than the excitation frequency used.

As a further alternative, the excitation winding may be wound as aninsulated wire parallel to the plane of the LCD display around thefolded portions of the flexible circuit board. This allows an increasedvolume of copper to be used thereby improving power efficiency and hencebattery life.

In the above embodiment, the flexible circuit board was arranged to fitover the front of the display and fold down around its sides. As analternative, the flexible circuit board may be arranged to fit over therear of the display and fold up around its sides. In this case, thecentral region of the circuit board may not require a window and may befitted with the additional sensor windings (such as those shown in FIGS.10 and 11). In this case, the bent up sides of the flexible circuitboard can include coils with axes parallel to the writing surface, againyielding improvements in accuracy and signal level near the circuitboard corners.

As those skilled in the art will appreciate, the outline of the flexibleprinted circuit board shown in FIG. 7 b can be replaced by anothercircuit board outline having other patterns of cuts and folds tosurround the display as appropriate when folded.

In the above embodiments, the novel excitation and/or sensor windingswere arranged so that their effective magnetic axes are substantiallyparallel to the plane of the LCD display. As those skilled in the artwill appreciate, it is not essential for these axes to be exactlyparallel with the plane of the LCD display. The axes preferably lie atan angle of between 0 and 5° to the plane of the LCD. Further, theinventor has found that improvements in efficiency are still achievedwhen the axes of the windings lies at an angle between 0 and 600 to theplane of the display.

In the embodiment described above, an excitation operation was performedfollowed by a detection operation. As those skilled in the art willappreciate, it is not essential for the detection operation to beperformed after the excitation operation. For example, the detectionoperation may begin before the excitation operation has ended, althoughthis is not preferred due to potential coupling between the excitationwindings and the sensor windings, which may result in errors in theposition measurements.

In the above embodiment, the system was operated in a pulse echo mode inwhich the excitation winding(s) is energised and then the signals in thesensor windings are processed. However, it is possible to operate thesystem in a continuous mode of operation (where at least one of theexcitation windings is continuously energised) provided it is possibleto distinguish the return signal from any excitation breakthrough. Thiswill be the case if the stylus uses a resonator since the resonatorsignal will be electrically in phase quadrature with the breakthroughsignal. This will also be the case with some other types ofelectromagnetic device, such as harmonic generators or electronictransponders that transmit at a different frequency to the excitationfrequency or which radiates digitally coded signals etc.

In the above embodiment, a particular arrangement of processingelectronics is described. As those skilled in the art will appreciate,the signals generated in the sensor windings may be processed by anyappropriate processing electronics which can derive the requiredinformation from the received signals. As illustrated in FIG. 15 a, inthe general case, the processing electronics will include a system model241 which relates the way in which the received sensor signals varieswith the parameters to be measured. In the particular embodimentdiscussed above, the parameters to be measured may include one or moreof the x, y, z position of the stylus, the tilt of the stylus, therotation of the stylus and the status of the stylus. In the embodimentsdescribed above, the system model 241 was effectively the sin and cosrelationships defined in equations 1 to 8. As an alternative to applyingthe signal measurements to predetermined equations (and as illustratedin FIG. 15 b), the processing electronics may include a field model 253which models the field patterns which will be generated in the systemfor a given set of stylus parameters and uses these field patterns topredict the values of the signal measurements. The predicted values ofthe signal measurements are then compared with the actual signalmeasurements in a signal comparator 255 and the results used to updatethe estimation of the stylus parameters. This process is then repeateduntil the error between the predicted signal measurements and the actualsignal measurements is minimised or reaches some convergence criteria.

In the main embodiment described above, the signals induced in thesensor windings were passed through respective processing channelscomprising a mixer and an integrator. As those skilled in the art willappreciate, the mixing and integration process may be performed in thedigital electronics, with the raw sensor signals being fed directly intothe analogue-to-digital converter. However, such an embodiment requiresmore complex digital electronics. Additionally, the signals from thedifferent sensor windings may be time-multiplexed through the sameprocessing channel in order to reduce the number of system components.

In the above embodiment, the resonant stylus included a passive resonantcircuit. As those skilled in the art will appreciate, different types ofstylus may be provided for interacting with the sensor and/or excitationwindings. For example, the resonant stylus may be replaced by a shortcircuit coil, a piece of ferrite, a mechanically resonant device such asa magneto-strictive element, a conductive screen etc. It is alsopossible to include electronics within the stylus, with the magneticfield generated by the excitation winding being used to power theelectronics in the stylus. Multiple resonators could also be used in thestylus which can provide more information about the status of thestylus.

In the above embodiment, the stylus was arranged so that the resonantfrequency of the stylus changed with pressure applied to the tip. In analternative embodiment, one or more switches may be provided on thestylus which may be actuated by a user in order to change the resonantfrequency of the stylus. This can then be detected by the processingelectronics in order to exchange status information between the stylusand the processing electronics.

In the above embodiment, the excitation and processing circuitry wasformed in the same device as the excitation and sensor windings. Asthose skilled in the art will appreciate, the excitation and processingcircuitry may be provided on a remote body from the sensor windings. Allthat is required is that the resonant stylus be energised by anappropriate energising field and for the signals received in the sensorwindings to be transmitted to the processing circuitry.

In the above embodiment, a single stylus was provided. As those skilledin the art will appreciate, the system may operate with multiplestyluses each having their own characteristic (e.g. resonant frequency)so that the system can differentiate the styluses being used. Eachstylus may then be assigned a different function in the system.

In the above embodiments, the windings were arranged over a generallyrectangular measurement area corresponding to the x-y display. As thoseskilled in the art will appreciate, this is not essential. The sensorwindings and the excitation windings may be arranged overnon-rectangular areas.

In the above embodiment, each of the sensor windings was formed usingmultiple turns of conductor. As those skilled in the art willappreciate, the sensor windings can be formed using a single turn ofconductor. However, this is not preferred since the sensor winding'ssensitivity to the magnetic field generated by the resonator is lesssinusoidal and the signal levels output are smaller. It is thereforepreferred to have as many turns as possible in the sensor windings.

In the above embodiments, the stylus was inductively coupled to both theexcitation windings and the sensor windings. As those skilled in the artwill appreciate, it is not essential to have inductive coupling betweenboth the stylus and the excitation windings and the stylus and thesensor windings. For example, the stylus may be inductively coupled tothe excitation winding and capacitively or electrostatically coupled tothe sensor windings or vice versa. Alternatively, the stylus may bearranged to transmit a RF signal to an appropriate receiver oncepowered.

1. A signal transferring apparatus comprising: first and second memberswhich are relatively movable over an x-y planar working area; the firstmember comprising a coil whose effective magnetic axis is non-orthogonalto the x-y planar working area; and the second member comprising anelectromagnetic device operable to interact with said coil to exchange asignal between said first and second members.
 2. An apparatus accordingto claim 1, wherein said first member further comprises a drive circuitoperable to apply a drive signal to said coil to create a magnetic fieldover said x-y planar working area and wherein said electromagneticdevice of said second member is operable to be energised by saidmagnetic field.
 3. An apparatus according to claim 2, wherein saidelectromagnetic device comprises a coil which is operable to couple withsaid magnetic field to energise said electromagnetic device.
 4. Anapparatus according to claim 2, wherein said electromagnetic devicecomprises a resonator.
 5. An apparatus according to claim 1, whereinsaid electromagnetic device is operable to generate an electromagneticfield which interacts with said coil to generate one or more sensorsignals and wherein said first member further comprises a processoroperable to process said one or more sensor signals.
 6. An apparatusaccording to claim 5, wherein said one or more sensor signals vary withthe status of said electromagnetic device and wherein said processor isoperable to process said one or more sensor signals to determine thecurrent status of the electromagnetic device.
 7. An apparatus accordingto claim 6, wherein said status comprises the current position of thesecond member relative to the first member and wherein said processor isoperable to process said one or more sensor signals to determine thecurrent position of said second member relative to said first member. 8.An apparatus according to claim 5, wherein said one or more sensorsignals are generated in said coil.
 9. An apparatus according to claim5, wherein said first member comprises one or more additional coils andwherein said one or more sensor signals are generated in said one ormore additional coils.
 10. An apparatus according to claim 9, whereinsaid one or more additional coils are formed as conductor tracks on aprinted circuit board.
 11. An apparatus according to claim 10, whereinsaid circuit board lies predominately within a plane parallel to saidplanar working area and comprises foldable portions which, in use, lieat an angle to said plane and wherein one or more loops of at least oneof said additional coils are formed on said foldable portions.
 12. Anapparatus according to claim 11, wherein said foldable portions are atan edge of said circuit board.
 13. An apparatus according to claim 11,wherein said circuit board comprises a transparent window.
 14. Anapparatus according to claim 13, wherein said first member comprises adisplay and wherein said circuit board is arranged to fit over or undersaid display and wherein said foldable portions fold along the sides ofthe display.
 15. An apparatus according to claim 1, wherein said firstmember comprises a plurality of coils whose effective magnetic axes arenon-orthogonal to the x-y planar working area.
 16. An apparatusaccording to claim 1, wherein the effective magnetic axis of saidwinding lies at an angle of between zero and 60E to the plane of saidx-y planar working area.
 17. An apparatus according to claim 1, whereinthe effective magnetic axis of said coil is substantially parallel tothe x-y planar working area.
 18. An apparatus according to claim 1,wherein said first member comprises one or more additional coils whoseeffective magnetic axes is substantially orthogonal to the x-y planarworking area.
 19. An apparatus according to claim 18, wherein said oneor more additional coils are formed over a peripheral portion of saidx-y planar working area.
 20. An apparatus according to claim 18, whereinsaid one or more additional coils are arranged to have a sensitivityfunction to magnetic field which spatially varies approximatelysinusoidally across at least one dimension of said x-y planar workingarea.
 21. An apparatus according to claim 1, wherein said coil comprisesfirst and second coil portions which are located on opposite edges ofthe x-y planar working area and which are connected in series to have asensitivity function which spatially varies approximately sinusoidallyacross the planar working area.
 22. An apparatus according to claim 1,wherein said first member includes an x-y circuit board arrangedsubstantially parallel to said x-y planar working area and wherein saidcoil is wound around at least a part of said planar circuit board sothat its effective magnetic axis substantially lies in a plane parallelto said working area.
 23. An apparatus according to claim 1, whereinsaid first member comprises a layer of high magnetic permeabilitymaterial arranged substantially parallel to said x-y planar workingarea, and wherein said coil is wound around at least a part of saidmagnetic material such that its effective magnetic axis substantiallylies in a plane parallel to said working area.
 24. An x-y positionsensor comprising: first and second members which are relatively movableover an x-y planar working area; the first member comprising a coilwhose effective magnetic axis is non-orthogonal to the x-y planarworking area; the second member comprising an electromagnetic deviceoperable to interact with said coil to transfer a signal between saidfirst and second members, which signal varies with the relative positionof the first and second members; and a processor operable to processsaid signal to determine the relative position of the first and secondmembers.
 25. A portable computing device comprising: a planar display;an x-y digitiser associated with the display and operable to sense thestatus of an indicator which is moveable relative to the display; and aprocessor operable to control the display in accordance with the sensedstatus of said moveable indicator; and wherein said x-y digitisercomprises at least one winding whose effective magnetic axis isnon-orthogonal to the plane of said display.
 26. An x-y position sensorcomprising: first and second members which are relatively movable overan x-y planar working area; the first member comprising a plurality ofwindings arrayed over the x-y planar working area and each windingcomprising at least two loops wound in opposite senses; the secondmember comprising an electromagnetic device operable to interact withsaid plurality of windings to generate signals that vary with therelative x-y position of the first and second members; and wherein theloops of said plurality of windings are arranged in at least two rowsand at least two columns, with loops in the same column being wound inthe same sense and with adjacent loops in the same row being wound inalternating sense.
 27. A sensor according to claim 26, wherein the loopsof each winding are positioned diagonally within said rows and columns.28. A sensor according to claim 26, wherein each winding includes atleast one loop in each row and at least one loop in each column.
 29. Asensor according to claim 28, wherein the loops of each winding arelocated in alternating positions of said rows and columns.
 30. A planarcircuit board for use in an x-y position sensor, the circuit boardcomprising: a plurality of windings arrayed over the board, each windingcomprising at least two loops wound in opposite senses and wherein theloops of said plurality of windings are arranged in at least two rowsand at least two columns, with loops in the same column being wound inthe same sense and with adjacent loops in the same row being wound inalternating sense.
 31. A circuit board for use in a position sensor, thecircuit board comprising: a central portion having one or more conductorloops and a flexible peripheral portion which has one or more conductorloops and which can flex relative to said central portion.
 32. A circuitboard according to claim 31, wherein said central portion includes atransparent window and wherein said conductor loops of said centralportion are formed at at least one edge of said transparent window. 33.A circuit board according to claim 32, adapted to fit over a display,with the display being viewable through said transparent window and saidflexible portion being arranged to fit over the sides of said display.34. A method of transferring signals comprising the step of using anapparatus according to claim
 1. 35. An x-y position sensor comprising:first and second members which are relatively movable over an x-y planarworking area; the first member comprising a plurality of windingsarrayed over the x-y planar working area and each winding comprising atleast two loops wound in the opposite sense; the second membercomprising an electromagnetic device operable to interact with saidplurality of windings to generate signals that vary with the relativex-y position of the first and second member; and wherein theelectromagnetic coupling between said electromagnetic device and atleast one of said windings varies with both the relative position ofsaid first and second members in both the x and y directions.
 36. An x-yposition sensor comprising: first and second members which arerelatively movable over an x-y planar working area; the first membercomprising a circuit board arranged substantially parallel to saidplanar working area and comprising at least one winding arranged in a xdirection and at least one winding arranged in a y direction; the secondmember comprising an electromagnetic device operable to interact withsaid windings to generate signals that vary with the relative x-yposition of the first and second members; and wherein said windings arearranged on a peripheral edge of said circuit board and wherein acentral portion of said circuit board is free of windings.
 37. A sensoraccording to claim 36, wherein said central portion of said circuitboard includes a transparent window.
 38. A portable computing devicecomprising: a display; a x-y digitiser associated with the display andoperable to sense the status of an indicator which is movable relativeto the display; and a processor operable to control the display inaccordance with the sensed status of said movable indicator; and whereinsaid x-y digitiser comprises at least one winding whose effectivemagnetic axes is non orthogonal to the plane of said display and isoperable to power said indicator when operating above the display andwhen operating to a side of the portable computing device.
 39. A deviceaccording to claim 38, wherein said indicator has a first mode ofoperation in which the indicator interacts with said x-y digitiser and asecond mode of operation in which it acts as a conventional mouse and ispowered by said at least one winding of said x-y digitiser.